The present invention relates to an oblique projection optical system, for example, to an oblique projection optical system suitable for use in an image projector and an image reader.
A projector that projects an image displayed on a liquid crystal display or the like onto a screen while enlarging it has been known. With the objective of reducing the overall size of a projector while increasing the screen size, various types of projectors that project an image from a slanting direction have been proposed. Oblique projectors of this type in which the projection optical system is disposed away from the center of the screen also produce an advantage that the projector does not obstruct the user""s view.
However, in the oblique projectors, a so-called trapezoidal distortion occurs in projected images. Moreover, in view of user convenience, a function that makes the enlargement magnification variable (that is, zoom function) is further desired. A projector in which some contrivance is given to the lens moving method in order to achieve correction of the trapezoidal distortion and provision of the zoom function is proposed in Japanese Laid-open Patent Application 5-119395. In this prior art application, however, there is no specific disclosure of the optical structure, and according to the optical structure proposed in the prior art application, it is difficult to reduce the size of the projection optical system and secure a sufficient angle of oblique projection.
An object of the present invention is to provide an improved oblique projection optical system.
Another object of the present invention is to provide a compact and high-performance oblique projection optical system in which a sufficient angle of oblique projection can be secured, said oblique projection optical system being a practical wide-angle and high-magnification zoom optical system.
The above-mentioned objects are achieved by an oblique projection optical system having the following structure:
An oblique projection optical system performing enlargement projection in a slanting direction from a primary image plane to a secondary image plane, or reduction projection in a slanting direction from the secondary image plane to the primary image plane, is provided with at least two lens units each comprising at least one lens element and having surfaces decentered with respect to each other. Zooming is performed by moving, of the lens units, a plurality of lens units. When a ray connecting the center of the primary image plane and the center of the secondary image plane is referred to as an image plane center ray, the following conditions (1) and (2) are satisfied without an intermediate real image being formed between the primary image plane and the secondary image plane:
5xc2x0 less than |xcex8oi| less than 40xc2x0xe2x80x83xe2x80x83(1)
20xc2x0 less than xcex8o less than 50xc2x0xe2x80x83xe2x80x83(2)
where
|xcex8oi| is the angle between the primary image plane and the secondary image plane; and
xcex8o is the angle between the normal to the secondary image plane and the image plane center ray in a condition where the magnification on the primary image plane side is lowest [W].